1. Field of the Invention
The present invention relates to a zoom lens system which is to be used with video cameras and so on.
2. Description of the Prior Art
As solid-state image pickup devices such as CCD's become more minute, higher imaging performance is required for zoom lens systems which are to be used with video cameras. For meeting demands for compactness of the video cameras themselves, on the other hand, it is desired to configure lens systems which are more compact and of lighter weight. It is generally known to use aspherical surfaces as means for obtaining enhanced performance and compacter design of zoom lens systems which are to be used with video cameras. Lens systems disclosed by Japanese Patent Kokai Publication No. Hei 4-301612, for example, are known as conventional examples which use an aspherical surface for configuring a lens system so as to have a high performance and a compact design. This lens system is a zoom lens system which is composed, in order from the object side, of a first positive lens unit, a second negative lens unit, a third positive lens unit, a fourth positive lens unit and a fifth negative lens unit; and moves the second lens unit and the fourth lens unit for changing magnification. However, this conventional example uses the aspherical surface in the fourth lens unit which is movable for changing the magnification, thereby requiring a manufacturing cost which is enhanced due to the aspherical surface requiring strict allowances for manufacturing.
As conventional examples of zoom lens systems which do not use aspherical surfaces in lens units that are movable for changing magnifications, there are known lens systems disclosed by Japanese Patents Kokai Publication No. Hei 4-78809, No. Hei 4-13109 and No. Hei 4-60509. Each of these conventional examples is composed, in order from the object side, of a first positive lens unit, a second negative lens unit, a third positive lens unit, a fourth positive lens unit and a fifth negative lens unit; moves the second lens unit and the fourth lens unit for changing a magnification, and uses an aspherical surface in the third lens unit or the fifth lens unit which is kept stationary during the change of the magnification. However, all of the lens units used in each of the conventional examples have strong refractive powers for configuring the zoom lens system to be compact and produce aberrations in large amounts, whereby the lens system as a whole cannot correct aberrations to favorable levels or exhibit imaging performance high enough to meet the demands posed by the more minute solid-state image pickup devices such as CCD's.